The transcription factor E2F plays a predominant role in regulating the transition from G1 to S phase of the cell cycle (La Thangue (1994) Trends Biochem. 19, 108-114; Müller (1995) Trends Genetics 11, 173-178). E2F is a heterodimeric factor consisting of one component of the E2F family and one component of the DP family. Six members of the E2F family and two members of the DP family have been identified in humans. While all members of the E2F family can form heterodimers with both DP members, they show a preferential binding to the different members of the pocket protein family. E2F1-3 interact with pRB, while E2F4 binds preferentially p107 and 130 but also pRB, and E2F5 only p130. Differences between the E2F members exist also in binding of cyclin A/cdk2 complexes. Binding sites for these complexes are found only in E2F1-3 (Beijersbergen & Bernards (1996) Biochem. Biophys Acta 1287, 103-130)